Molten copper or the like produced in a vertical furnace drains away through refractory lined "launders" into a holding furnace also having reducing gas burners. From the holding furnace, the copper is transferred to a water cooled casting wheel from which copper rod emerges.
When a vertical melting furnace is used with a holding furnace and at least one casting wheel, the objective is to keep the casting wheels busy all the time, but occasionally casting is stopped for any of several reasons. Since there is limited capacity in the rotary holding furnace, halting the casting wheel for more than a few minutes means that the flow of molten copper to the holding furnace must be stopped.
The standard industry practice is to "blow down" the vertical melting furnace with high flows of air to cool the melting copper below the melting point.
There are two reasons for having a rapid blow down: first to prevent overflow of the holding furnace, and second to prevent the soft, melting copper in the furnace from slumping to the bottom and congealing in a solid mass. If the latter occurs, several weeks are required to repair the furnace. Generally, the vertical melting furnace charge is a random mix of outside scrap copper, scrap rod being reprocessed, and so called "cathode" (pure copper plates produced to feed the vertical melting furnace). The temperature at the top of the vertical melting furnace is about 300.degree. F., and the bottom 2200.degree. F. There is normally less than 1" of molten copper in the bottom of the vertical melting furnace.
The problem when using air for blowing down the vertical melting furnace is that the entire exposed surface of the melting copper charge is oxidized by the air. Some oxidation occurs even at 250.degree. F., and the charge at the bottom of the furnace is grossly oxidized by using air to blow down the furnace. Furthermore, it is thought that significant oxidation may occur in less than 1 minute, perhaps in less than 15 seconds.
After an air blow down of one hour (a typical time period), the vertical melting furnace is restarted and the first molten copper produced is heavily oxidized. This copper flows into the holding furnace and is diluted. However, the effect of the introduction of excess oxides into the system is to produce copper rod with layers of oxide inside the rod (and hence not amenable to any later surface treatments). Internal layers of oxide cause the rod to be breakable, and to make unacceptable copper wire. Consequently, a normal one hour of scrap copper production follows an air blow down of the vertical melting furnace. This scrap is recycled back to be used as charge in the vertical melting furnace.